1. Field of the Invention
The device of the present invention generally relates to nuclear fuel assemblies and, more particularly, to new and improved apparatus and methods for providing coolant flow paths between separate fuel sections of a single nuclear fuel assembly.
2. Description of the Prior Art
The generation of a large amount of heat energy through nuclear fission in a nuclear reactor is old and now well known. This energy is dissipated as heat in elongated nuclear fuel rods. A plurality of the nuclear fuel rods are grouped together to form separately removable nuclear fuel assemblies. A number of such nuclear fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is typically submersed in a fluid, such as light water, that serves as a coolant for removing heat from the nuclear fuel rods and as a neutron moderator.
A typical nuclear fuel assembly may be formed by a 7.times.7 or an 8.times.8 array of spaced-apart, elongated rods supported between upper and lower tie plates. Examples of such typical nuclear fuel assemblies are depicted in and described in U.S. Pat. Nos. 3,350,275; 3,466,226 and 3,802,995. In a typical boiling water reactor nuclear fuel assembly having an 8.times.8 rod array, the sixty-four rods that form the 8.times.8 array may be either sixty-four fuel rods or one or more non-fueled, water moderator rods with the remaining rods being fuel rods. A common problem in typical boiling water reactor nuclear fuel assemblies of the types depicted in the above-identified patents is that the central region of such fuel assemblies may be undermoderated and overenriched. In order to increase the flow of moderator in the central region of such boiling water reactor nuclear fuel assemblies, one or more elongated, water moderator rods have been substituted for fuel rods in the central region of the nuclear fuel assemblies. For example, water moderator rods 41 and 42 are depicted in and described in the above-identified U.S. Pat. No. 3,802,995.
The flow of moderator may be increased, as disclosed in the above-identified Barry et al. application, an internal, centrally located or central water cross formed by four, elongated metal angles dividing the nuclear fuel assembly into four sections or quadrants along the length of the nuclear fuel assembly. A separate fuel bundle or subassembly is located in each of the four sections. The central water cross provides a region for subcooled moderator flow through the center of the nuclear fuel assembly along the lengths of the nuclear fuel rods for improving neutron moderation and economy. In addition, the central water cross enables a full complement of fuel rods to be used within the nuclear fuel assemby by eliminating the need for one or more water moderator rods as discussed above.
In the nuclear fuel assembly of this latter type flow paths are provided between the separate sections of the nuclear fuel assembly to provide hydraulic pressure equalization therebetween. The four angles forming the central water cross have numerous sealed passages formed therethrough to permit coolant flow between the separate sections of the nuclear fuel assembly. In accordance with the teachings of the prior art each sealed passage may be formed by perforating the walls of the four angles forming the central water cross and reforming the perforated material into a channel or passage which is then sealed from the interior of the central water cross, that is, the region of subcooled moderator flow, by one or more circumferential welds. While such coolant flow paths between the separate sections of the nuclear fuel assembly are effective in performing their intended function, manufacturing difficulties arise in forming many such flow paths within a single fuel assembly and in ensuring that no weld failures occur that could result in the mixture of boiling coolant from the separate sections of the nuclear fuel assembly with the subcooled moderator flowing in the central water cross.